1. Field of the Invention
The present invention relates generally to orthogonal frequency division multiplexed (OFDM)-based communications, and more specifically to tracking pilot tones of OFDM-based communications to reduce phase noise requirements in the radio portion of an OFDM receiver, as well as provide nearly optimal frequency error tracking performance.
2. Discussion of the Related Art
In wireless local area network (WLAN) applications, multiple devices communicate with each other via OFDM-based radio frequency (RF) wireless links. A common format for such OFDM communication is based upon the IEEE 802.11a standard or the HiperLAN2 standard, for example. Good local oscillator (LO) phase noise performance in the radio portion of the OFDM transmitters and receivers is critical in such OFDM-based communications when using complex signal constellations, such as 64-QAM and 256-QAM (quadrature amplitude modulation). This is because the symbol rate is chosen to be low enough to combat the severe multipath propagation characteristics that exist like those in indoor wireless applications and this low symbol rate also leads to greater phase noise related performance impairment. For example, in IEEE 802.11a and HiperLAN2, the symbol rate is approximately 250 kHz thereby accentuating the need to have excellent phase noise performance in the radio at frequency offsets from the carrier in the vicinity of 250 kHz and less.
Furthermore, the phase of the RF signaling is effected by phase noise generated in the local oscillators (LOs) of both the transmitter and the receiver. Also, phase perturbations are introduced when the transmitter or the receiver physically moves relative each other and also when the multipath changes, e.g., a door is opened. Unfortunately, poor LO phase noise performance leads to a potentially high symbol error rate, which seriously degrades both the communication range and throughput of the system. For example, in a typical system using IEEE 802.11a, it is estimated that the acceptable phase noise interfering with each subcarrier of the OFDM waveform is on the order of 2.7 degrees rms. While this may be acceptable for QPSK and 16-QAM modulations, it is excessive for 64-QAM modulation or higher constellations, resulting in constellation points being easily confused.
Further adding to the problem is the fact that most transmitters and receivers of such wireless products are highly integrated on a single device or chip. As such, the performance of the RF portion of the receiver, for example, is relatively limited. Furthermore, implementing the RF portion of the system to have the desired good phase noise performance that is required for higher order modulations, such as 64-QAM and above, is very difficult when implemented on a single chip with low supply voltages (e.g., 3.3 volts).
The present invention advantageously addresses the needs above as well as other needs by providing a pilot tracking system utilizing an optimum pilot phase error metric based on a maximum likelihood estimation approach in the baseband processing portion of the OFDM-based receiver to compensate for poor local oscillator performance in the radio portion of the OFDM-based receiver and transmitter and improve frequency tracking in general.
In one embodiment, the invention can be characterized as a pilot phase tracking loop for an orthogonal frequency division multiplexed (OFDM) receiver including a phase rotator for receiving and phase de-rotating an incoming signal, a fast Fourier transform coupled to an output of the phase rotator for processing a signal output from the phase rotator and a pilot phase error metric including a discrete Fourier transform portion. The discrete Fourier transform portion is coupled to the output of the phase rotator. The pilot phase error metric determines a phase error estimate associated with a received OFDM symbol of the signal output from the phase rotator. Also included are a loop filter coupled to an output of the pilot phase error metric and an oscillator coupled to an output of the loop filter. The oscillator has an output coupled to the phase rotator such that the phase rotator adjusts the phase of subsequent OFDM symbols of the incoming signal arriving after the received OFDM symbol by the phase error estimate.
In another embodiment, the invention can be characterized as a method of pilot phase tracking in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: receiving a baseband signal corresponding to an OFDM preamble waveform at a discrete Fourier transform portion of the OFDM receiver, wherein the discrete Fourier transform is a separate processing operation than a fast Fourier transform of the OFDM receiver; determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; receiving a baseband signal corresponding to an OFDM symbol at the discrete Fourier transform portion; determining complex signal measurements corresponding to each of the plurality of pilots of the OFDM symbol; determining a phase error estimate corresponding to the OFDM symbol based on the pilot reference points and the complex signal measurements; filtering the phase error estimate; and rotating a phase of an incoming signal corresponding to subsequent OFDM symbols to be received at the fast Fourier transform after the OFDM symbol by a filtered phase error estimate; wherein a phase noise of the incoming signal corresponding to the subsequent OFDM symbols to be received at the fast Fourier transform is reduced.
In yet another embodiment, the invention can be characterized as a method of pilot phase tracking in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: receiving a signal representing an OFDM waveform at a discrete Fourier transform portion of the OFDM receiver, wherein the discrete Fourier transform is a separate processing operation than a fast Fourier transform of the OFDM receiver that also receives the signal; determining a phase error estimate corresponding to an OFDM symbol of the OFDM waveform; filtering the phase error estimate; and rotating a phase of the signal for subsequent OFDM symbols to be received at the fast Fourier transform after the OFDM symbol by the filtered phase error estimate, wherein a phase noise of the signal for the subsequent OFDM symbols to be received at the fast Fourier transform is reduced.
In yet another embodiment, the invention can be characterized as a pilot phase error metric for an orthogonal frequency division multiplexed (OFDM) receiver including a discrete Fourier transform portion for receiving an incoming signal corresponding to an OFDM waveform. The discrete Fourier transform portion outputs complex signal measurements corresponding to each of a plurality of pilots of a preamble portion of the OFDM waveform and complex signal measurements corresponding to each of a plurality of pilots of a subsequent OFDM symbol of the OFDM waveform. The discrete Fourier transform portion is separate from a fast Fourier transform operation of the OFDM receiver. A maximum likelihood phase error/weighting processor is coupled to the discrete Fourier transform portion for processing the complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol in comparison to the pilot reference points. And a phase error estimator is coupled to the maximum likelihood phase error/weighting processor for estimating a phase error of the subsequent OFDM symbol relative to a phase corresponding to the preamble portion based on the processed complex signal measurements and the pilot reference points.
In a subsequent embodiment, the invention can be characterized as a method of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: determining pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; processing, in a parallel path to the determining step, the OFDM preamble waveform with a fast Fourier transform; determining a phase error estimate of a subsequent OFDM symbol relative to the pilot reference points; and processing, in the parallel path to the determining steps, the subsequent OFDM symbol with the fast Fourier transform; wherein the determining the phase error estimate step is completed prior to the completion of the processing the subsequent OFDM symbol with the fast Fourier transform in the parallel path.
In another embodiment, the invention can be characterized as a method of pilot phase error estimation in an orthogonal frequency division multiplexed (OFDM) receiver comprising the steps of: determining, in a separate processing path parallel to a fast Fourier transform processing path of the OFDM receiver, pilot reference points corresponding to a plurality of pilots of an OFDM preamble waveform; and estimating, in the separate processing path, an aggregate phase error of a subsequent OFDM symbol relative to the pilot reference points using complex signal measurements corresponding to each of the plurality of pilots of the subsequent OFDM symbol and the pilot reference points.